Hybrid Operation of Grid-following and Grid-forming Renewable Energy Sources for Commutation Failure Suppression of LCC-HVDC

With the integration of large-scale renewable energy sources (RESs), the line commutated converter (LCC) based high-voltage direct-current (HVDC) inverter side suffers from degraded grid strength and escalating commutation failure (CF) risks. In comparison with the widely used grid-following (GFL) RESs, grid-forming (GFM) RESs exhibit the favorable characteristics of voltage sources. Therefore, the hybrid operation of GFL-/GFM-RESs holds great potential in voltage support and may help CF suppression. This paper first analyzes the impacts of RESs on the first and subsequent CFs, respectively. It reveals that GFM-RESs alleviate the deterioration of grid voltage induced by GFL-RESs, thereby suppressing the first CF. However, current fault ride-through controls of RESs cannot fulfill the reactive power dynamic demand during the CF recovery process and rarely help mitigate subsequent CFs. Moreover, the limited overcurrent capability of GFM-RES hinders its ability to ride through CF. Based on the mechanism analysis, a CF suppression strategy based on the hybrid GFL/GFM operation is proposed. The proposed strategy optimized the controls of RESs to fulfill reactive power dynamics and offer voltage support during the CF process. Case studies are undertaken on the studied system and the CIGRE benchmark, respectively. The simulation results confirm the effectiveness of the proposed strategy for suppressing CF.